In a typical cellular radio communication system, an area is divided geographically into a number of wireless coverage areas such as cells and cell sectors, each defined by a radiation pattern from one or more base station antennae. As an access terminal moves between coverage areas, such as sectors, of a cellular radio communication system, or when network conditions change or for other reasons, the access terminal may “hand off” from operating in one coverage area to operating in another coverage area. In a usual case, this handoff process is triggered by the access terminal monitoring the signal strength of signals that it is receiving in various available coverage areas, and the access terminal or a radio network controller (RNC) determining when one or more threshold criteria are met.
In some systems, for instance, the access terminal may monitor signal strength in various available coverage areas and notify the RNC when a given coverage area has a signal strength that is sufficiently higher than the coverage area in which the access terminal is currently operating. The RNC may then direct the access terminal to hand off to that other coverage area. In other systems, for instance, the access terminal may monitor the signal-to-noise ratio in various available coverage areas and periodically notify the RNC which coverage area has the best signal-to-noise ratio and may thus offer the best service. The RNC may then effect a handoff to the designated coverage area by sending communications to the access terminal in that coverage area.
While this arrangement generally works well to facilitate seamless coverage as an access terminal moves between wireless coverage areas, a problem can arise when an access terminal is operating in or near a border area between coverage areas. When an access terminal operates in such an area, the access terminal may experience rapidly changing air interface conditions in the adjacent coverage areas, which may cause the access terminal to hand off back and forth between the coverage areas. For instance, when operating in the area of overlap between two sectors A and B and currently being served by sector A, the access terminal may detect that the signal-to-noise ratio in sector B is better and may thus hand off to sector B. But shortly after doing so, the access terminal may then detect that the signal-to-noise ratio in sector A is better and may thus hand off back to sector A.
This “ping-ponging” between sectors can be inefficient, since each handoff process may consume valuable system resources and delay communications to the access terminal. As a specific example, when an access terminal operating under the 1x Evolution-Data Optimized protocol (EV-DO or 1xEV-DO) seeks to switch to a new sector, switching sectors may cause delay in data transmission between the network and the access terminal that impacts throughput, as a backhaul connection (i.e., a connection carrying data to another connecting network, such as the Internet for instance) is established with the new sector, and as other network resources are put in place to serve the access terminal in the new sector. In addition, a network scheduler, which handles requests from access terminals and allocates network resources according to those requests, may create additional delay, depending on its treatment of a request from an access terminal that is switching sectors. In particular, additional delay may arise as the RNC waits to have sufficient data in its buffer to send to the access terminal in a next time slot. Consequently, an improvement is desired.